Foamed metal

ABSTRACT

FOAMED METALS HAVING IMPROVED CELLULAR STRUCTURE ARE PRODUCED BY MELTING A METAL, ADDING DROSS TO THE MOLTEN METAL, AND FOAMING THE MOLTEN BY MEANS OF A HEAT DECOMPOSABLE FOAMING AGENT.

United States Patent 3,671,221 FOAMED METAL Currie B. Berry, Jr., BatonRouge, La., assignor to Ethyl Corporation, New York, N.Y. No Drawing.Filed Oct. 30, 1970, Ser. No. 85,813 Int. Cl. B31d 3/00 U.S. Cl. 75-20 F8 Claims ABSTRACT OF THE DISCLOSURE Foamed metals having improvedcellular structure are produced by melting a metal, adding dross to themolten metal, and foaming the molten by means of a heat decomposablefoaming agent.

BACKGROUND OF THE INVENTION Field of the invention This invention is inthe field of metallurgy and, more specifically, pyrometallurgy.

Description of the prior art In the production of foamed metal, that is,metal having a plurality of randomly dispersed closed cells throughout ametal matrix, a preferred method is to use a heat decomposable foamingagent to generate the gas to foam the cells. This technique is disclosedin U.S. Pats. 2,751,289; 2,895,819; 2,983,597; 3,300,296; and 3,297,431.

Prior art foams frequently display non-uniform cellular structure. Thisproblem has been to some extent alleviated by incorporating metal oxideinto the foam. The metal oxide serves to thicken the molten metal andthereby assist the subsequent blowing step.

Unfortunately, employment of metal oxide normally adds to the cost ofthe foamed metal produced. While this cost is more than offset by theadvantages realized, the process would be even more attractive if suchcost could be reduced. Such reduction is a principle objective of thepresent invention.

SUMMARY OF THE INVENTION The present invention involves the use of drossin a foamed metal process as a source of metal oxide.

More particularly, the present invention concerns a process for foaminga molten metal or molten metal alloy where the improvement resides inmixing dross with the molten metal or molten metal alloy. Preferably,the dross is taken from a molten metal or molten metal alloy separatefrom that being foamed. A preferred metal is aluminum, andaluminum-magnesium alloys are especially preferred.

In accordance with the above-described procedure, the present inventionprovides several advantages which more than achieve the mentionedobjective of the invention. Thus, not only is the cost of the metal foamnot increased since the oxide source is dross which is normally a wasteproduce, but the metal content of the dross, which would normally belost, is recovered. This results in significant savings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The dross which is used inaccordance with this invention is a scum that forms on the surface of amolten metal. It is a waste or foreign matter which is usually disposedwithout recovery of its components.

Among the metals suitable for this invention, such as aluminum,magnesium, titanium, and the like, aluminum is preferred. Aluminumalloys are highly preferred and especially desirable are alloys of about96 to about 85 percent by weight aluminum with about 4 to about 15percent by weight magnesium.

The first step in the practice of the process of this invention is tomelt the metal or its alloy. This may be achieved with any suitableapparatus known in the art. It is desirable that such apparatus besusceptible to being maintained under an inert gas purge. Gases such asnitrogen are especially suitable for this purge. If a melting pot isemployed, the pot is raised to a temperature well above the temperatureat which the metal or its alloy normally becomes a liquid. Thisfacilitates quickly melting the metal or its alloy inasmuch as thetemperature is allowed to slowly recede as the metal or its alloybecomes completely liquid. Desirably the temperature of the metal or itsalloy is stabilized to the point of about 10 F. to about 50 F. above itsmelting point in order to insure that it will remain liquid during theremainder of the process.

Once the metal or its alloy has been liquified, it is agitated orstirred by means known in the art, e.g. an impeller, propeller, turbine,or the like. Where a turbine is employed, a rotation rate of r.p.m. to10,000 r.p.m. is suitable although 3,000 r.p.m. to 6,000 r.p.m. isgenerally preferred. It also appears useful to employ a slow rate ofstirring before the foaming agent, described hereinbelow, is added tothe molten metal or its alloy.

Once stirring or agitation of the molten metal or its alloy is wellunder Way, the dross is added to the molten material. Preferably thedross is drawn from other aluminum processing operations. The metaloxide furnished by the dross functions both as a viscosity increasingagent. While the mechanism by which the metal oxide increases theviscosity of the molten metal or molten metal alloy is unknown, it isspeculated that nucleation sites are created in the metal which functionto increase its viscosity.

The physical state of the dross is not critical inasmuch as thismaterial may be employed as a solid or liquid. Generally, it is mostconvenient to employ the material as a solid since it is solid atambient temperatures.

On the other hand, it is critical that the dross be uniformly admixedwithin the molten metal or its alloy. Thus, it is not enough to depositthe dross on the surface of the molten metal without mixing itthereinto. The dross must be blended uniformly into the molten materialby agitation or stirring, preferably at the rates above described.

It is suitable to use from about 0.01 or less to about 0.2 or more poundof the dross per pound of the metal or its alloy; preferably from about0.02 to about 0.1 pound of dross is employed for each pound of the metalor its alloy. More preferably, per each pound of the metal or its alloythere is employed from about 0.03 to about 006 pound of dross.

The dross preferably is added to the molten material at a rapid rate.The time period of addition may vary from about 2 seconds or less toabout 2 minutes or more and is subject to lengthening where very largequantities of dross are employed.

Generally, the pressure at which the dross is added to the molten metalor its alloy is not significant. Subatmospheric, super-atmospheric orambient pressures can be used but ambient pressure is preferred forreasons of economics.

Once the dross has been added, the metal or its alloy is now ready forthe foaming step. A wide variety of blowing agents can be used in thefoaming process of this invention. Broadly, all blowing agents describedin the prior art are suitable although some blowing agents are betterthan others. However, whatever the blowing agent, the foams of thisinvention have more uniform cellular structure than foams produced fromthe same metal or metal alloy which have not been mixed with dross.

Among the various blowing agents, the metal hydrides are preferred,among Which titanium, hafnium, or zirconium hydrides, especially thelatter, are most preferred. Dihydrides and annealed hydrides of lessthan stoichiometric composition also can be employed. Generally, thebest hydride blowing agents are those which decompose to yield gaseoushydrogen at the temperature of the metal or its alloy which is to befoamed and release hydrogen at a relatively slow rate.

The amount of foaming desired determines the amount of hydride or otherblowing agent employed; that is, for a dense foam less blowing agent isused than for a lighter foam. It generally is preferred to make foamshaving a 20 percent density or less, or to make foams which weigh nomore than about 20 percent of the Weight per given volume of theunexpanded metal. For such foams it is suitable to employ from about0.04 pound to about 0.4 pound of zirconium hydride for each pound ofmolten metal or its alloy to be foamed. A preferred range is from about0.08 pound to about 0.15 pound per pound of molten metal to be foamed.

In the foaming step, a temperature is employed which is above that atwhich the metal or its alloy to be expanded is molten and above thetemperature required to thermally decompose the blowing agent. Thetemperature, however, must not be so high that the blowing gas isreleased so fast as to cause foaming at an uncontrollable rate. Thus itis preferred to have the temperature of the molten metal or its alloycomparatively cool. Ideally, a temperature is employed at which the meltis barely viscous. Taking all these factors into consideration, themetal or its alloy is foamed at temperatures within the range from about1,130 F. to about 1,250 P. and preferably from about 1,150 F. to about1,200 E, being dependent of course upon the metal or its alloy used.

While it is suitable to carry out the foaming process at ambientpressure, greater or lesser pressures can be employed with betterresults under some circumstances. Lower pressures can be deleterioussince they can encourage evolution of blowing gas outside the confinesof the mass to be foamed. Super-atmospheric pressures up to 1,500p.s.i.g. or higher can be used.

In conducting the blowing step above described, the foaming agent ispreferably admixed with the molten metal or its alloy to be foamed byusing the agitating or stirring means earlier set forth. In the courseof such I stirring or agitation, the rate is preferably increased abovethe initial agitation rate at which time the foaming agent is added.Without exception, the more uniform the mixing, the better the foam. Alltechniques of mixing known in the art which ensure efiicient mixing ofmaterials and liquids can be employed. Preferably the mixing step isperformed in as short a time as is feasible to achieve uniform mixing.For best results with a typical mixture of blowing agent and moltenmetal or its alloy, sufficient mixing achieves homogeneity within aboutseconds. This time period may require an agitation rate with a stirringdevice of up to 10,000 rpm.

-As a rule it is preferable that the addition of foaming agent be at alower temperature than the addition of the dross. Accordingly, it ispreferred to cool the viscous metal before adding the foaming agent.Frequently, the cooling is best carried out in a second vessel, i.e., avessel other than the hot chamber in which viscosity was increased. Thesecond vessel is preferably generally to 4 within plus or minus C.,preferably plus or minus 20 C. of the foaming temperature, whereupon theviscous material is added thereto.

Subsequent to the addition of the blowing agent, the molten metal or itsalloy is allowed to foam. Foaming may occur within an open or closedmold. The size of closed foaming chambers relative to the quantity ofmetal or its alloy determines density of the product. Regulation of themold temperature determines the smoothness and thickness of the skin onthe finished article.

Having thus described the invention, the following example is presentedonly as being illustrative and not as being limiting of the invention.

EXAMPLE Two hundred pounds of a molten aluminum alloy at about 1,200" F.with 7 percent magnesium is poured into an agitated vessel. Four poundsof dross which is normally discarded from another aluminum processingoperation is added to the vessel. The viscosity is then increasedslightly by agitating the mixture in the presence of air. Two pounds ofzirconium hydride is then added and the mixture vigorously agitated forfifteen seconds, after which the foaming mixture is immediatelytransferred to a mold. Significant raw materials are saved by theprocess.

I claim:

1. In a process for foaming a molten metal or molten metal alloy,comprising melting the metal or metal alloy and mixing a blowing agenttherewith for foarming the metal or metal alloy, the improvementcomprising mixing dross with said molten metal or molten metal alloyprior to mixing the blowing agent, said dross being taken from a moltenmetal or molten metal alloy which is separate from said molten metal ormolten metal alloy being foamed.

2. The process of claim 1 wherein said molten metal is aluminum.

3. The process of claim 1 wherein said molten metal alloy is analuminum-magnesium alloy.

4. The process of claim 3 wherein from about 0.01 to about 0.2 pound ofdross is added per pound of aluminum-magnesium alloy.

5. The process of claim 3 wherein said aluminummagnesium alloy is heatedto from about 1,150 to about 1,250 F. before said dross is added.

6. The process of claim 3 wherein said dross is intimately mixed withsaid aluminum-magnesium alloy by a turbine at from about 500 rpm. toabout 8,000 r.'p.m. for from about 2 seconds to about 2 minutes.

7. The process of claim 3 wherein said molten metal or molten metalalloy is foamed within 30 minutes after said dross is added.

8. The process of claim 1, wherein said dross is taken from a moltenaluminum or aluminum-magnesium alloy.

References Cited UNITED STATES PATENTS 3,305,902 2/1967 Bjorksten -20 FX 1,919,730 7/1933 Koenig et al. 7520 F 3,379,517 4/1968 Graper 7520FOREIGN PATENTS 729,339 5/ 1955 Great Britain 75-20 F 811,814 4/1959Great Britain 7520 F L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU,Assistant Examiner

